There is a need for a continuous process for separating tertiary amine hydrochloride impurities from water immiscible polymer solutions of a halogenated aromatic polyester containing high concentrations of polymer solids which are therefore highly viscous. For example, when halogenated aromatic polyesters are prepared by the solution polymerization technique, as disclosed, for example, in U.S. Pat. No. 3,234,167, which is hereby incorporated by reference, there may be present in the reaction mixture large amounts of triethylamine hydrochloride both solubilized and suspended. This triethylamine hydrochloride must be removed from the polymer product. The reaction mixture, however, is a rather viscous solution having a viscosity of generally more than about 50 poise.
Although countercurrent extraction has been used to remove impurities from comparatively non-viscous solutions, the countercurrent extraction technique has not been applied to viscous solutions (i.e., solutions having a viscosity of more than about 10 poise) since high viscosity solutions do not readily form small droplets. The water-soluble impurities cannot therefore be readily extracted from large droplets having a relatively low surface area because transfer of water-soluble impurities through such large droplets is much less efficient than it is through small droplets. Furthermore, the large difference in viscosity between the polymer solution and the water extractant hinders the diffusion of the hydrochloride from the organic phase to the aqueous phase and complicates the extraction process even further. A commercial countercurrent extraction process also requires that the tertiary amine hydrochloride be economically recovered from the extract so that it can be reused in further processing operations.
Prior art methods for removing triethylamine hydrochloride from viscous solutions of halogenated aromatic polyesters in solvents such as methylene chloride comprise multiple batch extractions with a volume of water equal to the dope volume for each extraction. Such multiple batch extractions are time-consuming and uneconomical because of both capital expense and lack of recoverability of the triethylamine hydrochloride. Furthermore, multiple batch extractions are impractical for a production operation.
The search has continued for a continuous countercurrent extraction process for removing water-soluble tertiary amine hydrochloride impurities from viscous polymer solutions of halogenated aromatic polyesters. This invention was made as a result of that search.